Continuity mistake: In the scene where Belle first comes to the gate castle to find her father's hat, she pushes the left side of the gate that's already partly open. But when her father came, he opened the right side of the gate and slammed it shut. How did the left side of the gate open? (00:20:05)
Continuity mistake: When the Beast is getting ready, just before the ballroom scene, the coat stand is cutting the Beast's hair with a pair of scissors that has red handles. Then, in the final shot, the whole pair of scissors is silver. (00:59:40)
Continuity mistake: In the beginning of the movie, Gaston is using a blunderbuss (early rifle) to shoot the birds. He also has a quiver of arrows on his back. But he doesn't have a bow until the end of the movie.
Continuity mistake: When the Beast shows the mirror to Belle she is reflected in it, but for the rest of this scene she isn't.
Continuity mistake: When Gaston says "Now when Belle and I come out that door", you can tell that he has no instrument, only the musicians do. But in the next shot, Gaston has a tuba in his hands, and he slams it on top of Lefou's head.
Continuity mistake: When the Beast is shown falling into despair after scaring Belle away from the West Wing, the pieces of rock on the ground have all either changed position or appeared out of nowhere from the shot when Belle first saw the enchanted rose.
Continuity mistake: After Chip taps the spoon its position changes between shots.
Continuity mistake: When Cogsworth falls down the stairs while trying to stop Maurice from going into the room, a lot of his clockwork falls out and lands on the stairs. But in the next shot, it's all gone.
Continuity mistake: In the scene where the villagers try to imprison Maurice, there is a lot of snow on the stairhandle. When Belle comes out later crying "My father's not crazy", the snow is gone.
Continuity mistake: When Belle enters the West Wing, there are no candles next to the door. Later, Gaston enters with his bow, and now there are several candles next to the door.
Continuity mistake: During the fight scene at the end, knives are thrown at the villagers. Nobody is hit, but three knives get stuck in the left kitchen door. The villagers open both kitchen doors and, from the inside of the kitchen door, there are only two knives, each stuck in the right kitchen door.
Continuity mistake: When the Beast is waiting for Belle to come to dinner, there is a fireplace with statues of lions on either sides and a pillow next to one of them. However, a few seconds later, the pillow is now in front of the statue.
Continuity mistake: In the scene where Maurice's Rube Goldberg-ish woodcutting invention works for the first time, we see the piece of wood fly over onto the stack. There is no window there or anything made of glass for that matter, yet in the next shot we hear glass breaking from the wood flying over there.
Continuity mistake: At the end of the Provincial Life song, Gaston is singing from among the crowd and he doesn't have his gun. He climbs up the side of the building without his gun, but when he pops out of the window onto the roof he has his gun. When he pops in front of Belle on the street, the gun is gone again. (00:07:05)
Continuity mistake: Belle's nails disappear and reappear throughout the movie.
Continuity mistake: When Beast is grieving in the rose room before Gaston shows up, the chair has a missing piece of fabric on the left side. When Gaston shoots the arrow, the fabric is now fixed.
Continuity mistake: In the opening song sequence, there are three blonde girls singing about Gaston next to a water pump. But in a front shot of Gaston walking through the city as they sing, it is a barrel, not a pump, that they are gathered around.
Continuity mistake: There are 2 different designs for Gaston's chair that alternate all the time: one covered in dark fur with V-shaped antler armrests, and another with a lighter coloured fur with S-shaped antler armrests. Also, both antlers on each side of the backrest differ slightly.
Continuity mistake: When Maurice first enters the castle you can see that the carpet leading to the staircase is very long. When Belle enters the carpet is shorter.
Continuity mistake: When the Bimbettes sing "Oh, he's so cute!", the hand-pump lever is lowered, only to reappear raised one frame later.
Answer: The Rose started wilting at The Beast's 21st birthday and could have presumably started wilting years before the first time we saw a petal fall off. Given the fact that Lumiere stated that they were living with the curse for 10 years, prince Adam could be 23-24 years old at the time of the original movie,13-14 at the time of the curse, and the Rose could have been wilting for two to three years. This is my best guess given the approximate age of Prince Adam in the ripped up portrait before he was the Beast.